The invention relates to an exhaust air dryer with a drying chamber for items to be dried, which has a process air fan and at least one heat exchanger, wherein the drying chamber, the process air fan and the heat exchanger are interconnected by means of air ducts for the conveying of process air.
Such an exhaust air dryer follows from DE 30 00 865 A1.
Dryers for items of laundry and items of a similar kind are generally embodied as exhaust air dryers or condensation dryers. In the case of exhaust air dryers a stream of air is sucked in from the environs of the dryer, heated, directed over items to be dried and subsequently expelled from the dryer as “exhaust air”. This exhaust air contains all the moisture removed from the items to be dried, and can therefore not simply be released into the building, as this moisture would precipitate out therein; rather, the exhaust air must be directed out of the building by means of a corresponding exhaust air hose. This is a constructive disadvantage of the exhaust air dryer, which is at the same time of great structural simplicity and thus low in cost. A condensation dryer, whose method of functioning relies on the condensation of the moisture evaporated from the items to be dried by means of process air conducted in a closed circuit, requires no hose for expulsion of the moisture-laden process air, as the moisture condensed therein is stored as liquid, and disposed of after completion of the drying, and can thus be used in an internally located utility room or inside laundry room of a larger residential complex. All this applies both to dryers intended specifically for drying laundry and to so-called washer/dryers, that is appliances capable of both washing and drying laundry. Any subsequent reference to a “laundry dryer” or simply “dryer” thus applies both to a device intended for drying and to one designed equally for washing and drying.
Both in a conventional exhaust air dryer and in a conventional condensation dryer, the heat fed to the process air is largely lost. In an exhaust air dryer, the heat is carried away with the process air laden with moisture from the items to be dried, while in a condensation dryer the heat reaches a cooling medium, generally cooling air from the environs of the dryer via a heat exchanger, and is thus equally lost.
In a laundry drying device equipped with a heat pump, the cooling of the warm, moisture-laden process air and the condensing out of the moisture contained essentially take place in a first heat exchanger of the heat pump, which forms a heat sink, as heat is taken into the heat pump via said heat sink. From the heat sink, the heat pump pumps the absorbed heat into a second heat exchanger, a heat source, where the pumped heat, along with additional heat generated during operation of the heat pump, is given off again. The heat sink is in particular an evaporator, where the transferred heat is used to evaporate a coolant circulating in the heat pump. Such coolant, which is evaporated as a result of the heat, is fed, via a compressor, to the heat source, in this case hereinafter referred to as “condenser”, where heat is given off through condensation of the gaseous coolant, which is in particular in turn used to heat the process air before it comes into contact with the items to be dried. The liquefied coolant returns to the evaporator through a throttle element, which reduces its pressure, in order there to evaporate subject to the further absorption of heat from the process air. Compressor-combinations, as previously described, are employed as customary heat pumps. As a rule, these operate optimally within a specific temperature range. Other types of heat pumps are conceivable, in particular heat pumps which make use of the Peltier effect, a regenerative gas circuit or a sorption effect.
A combination system for heat recovery in an exhaust air dryer is known from the document DE 30 00 865 A1, which is a simple air-to-air heat exchanger. Here, heat is removed from the exhaust air in the heat exchanger, and fed to the inflowing supply air, which as a rule flows into the heat exchanger surfaces in ambient conditions (e.g. 20° C. and 60% relative air humidity), and is thus pre-heated before reaching a resistance heating unit and the laundry to be dried.